The activity of the aryl hydrocarbon hydroxylase of rat liver and of cultured rat liver cells is stimulated by pre-exposure of the rat or cultured cells to particular carcinogens. It is the purpose of this project to investigate the mechanisms by which this induction occurs. As the first stage of this investigation, electrophoretic estimations were made of the levels of the various microsomal proteins in rat liver, after treatment of the rats with the carcinogenic polycyclic hydrocarbon, 3-methylcholanthrene. These results were compared with those found after phenobarbital treatment. With the carcinogen two microsomal proteins were induced concomitantly in several strains of rat, and there was evidence suggesting that a third microsomal protein was decreased in amount. Increases in some microsomal proteins were also found after phenobarbital treatment, but these proteins were of somewhat lower molecular weight than those produced in response to the carcinogen. Similar investigations of the induction process are now being made in vitro, in slices of rat liver and in cultured diploid rat liver cells.